Refractory metal articles



March 11, 1969 G. LLEWELYN TAL 3,432,280

REFRACTORY METAL ART I CLES Filed Feb. 19, 1965 I NVENTORS ATTORNEYS UIlltCCl 51121168 .II' 211261112 UIUC@ 0,1J,40u Patented Mar. 11, 1969 6,954/ 64 U.S. Cl. 29--197 8 Claims Int. Cl. C23c 17/00 ABSTRACT F THE DISCLOSURE A refractory article composed of at least 50% tantalum is provided with a protective surface layer by heating it in contact with a powdery mixture comprising chromium to 50%, silicon 20% to 75% and aluminum 3% to 25% under time .and temperature conditions to form a layer at least 0.001 inch thick, in the presence of an inert gas.

Our British Patent No. 950,119 relates to a method of providing a protective surface layer on articles made from niobium and niobium base alloys to provide a form of surface protection which will increase the life of such articles when operating in a temperature range of 1100 C. to 1300 C. to a practically useful value.

It has now been found that [articles made from tantalum and tantalum base alloys can be protected by surface layers formed in the same way.

According to this invention, an oxidation-resistant refractory article comprises a body of refractory metal in which tantalum constitutes at least 50% yby weight, and a modified surface layer, preferably at least 0.001 inch thick, containing chromium, silicon and aluminium alloyed with the body met-al.

The invention also includes a method of making an article as defined in the preceding paragraph wherein an article made from the :refractory metal is heated in contact with a powdery mixture comprising chromium, silicon and aluminium in powder form in admixture with a non-reactive refractory dispersant and an activating substance selected from the group consisting of ammonium bromide, yammonium chloride and ammonium iodide, the time and temperature of the heating being suicient to -form on the article a modified surface layer at least 0.001 inch thick containing chromium, silicon and aluminium alloyed with the body metal, and the powdery mixture being permeated with an inert gas throughout the heating.

The single gure of the drawing shows an embodiment of the article.

The reactive content of the powdery mixture (that is to say, the chromium, silicon and aluminium) preferably comprises by weight, 20% to 75% chromium, 20% to 75 silicon and 3% to 25 aluminium, the particle size in each case being up to 300 mesh per inch (that is to say, the particles are of any size up to that at which they can just pass through a BSS 300 mesh per inch sieve). Preferably the content of aluminium is below 10% by weight of the total metal content.

The non-reactive refractory dispersant is preferably 'alumina having a particle size of from 300 to 100 mesh per inch and is present in a quantity by Weight which may vary from to 70% of the weight of the powdery mixture, and is preferably between 25 and 50%.

The `activating substance is preferably ammonium chloride, present to the extent by weight of from 0.5% to 2% of the total weight of the other ingredients of the powdery mixture, while the permeating gas is preferably argon or helium, neither nitrogen nor hydrogen being suiciently inert for the materials concerned.

Heating Lrnay be carried out in one stage at a constant temperature. Preferably however it is carried out in two stages of which the first stage is at a lower temperature. The temperature during the first stage can then be set Iat a value, preferably between 1100 C. and 1200* C., at which any small traces of oxygen or nitrogen which may be present as impurities in the permeating gas will lbe absorbed into the body metal only to a negligible extent. At the same time 'a thin protective layer will be formed on the body metal and this will protect the body metal against oxygen and nitrogen during the second stage of heating, which is preferably at between 1300" C. and 1400 C.

In the case of heating being carried out in one stage, the reactive content of the powdery mixture preferably, has, by weight, 40% to 70% chromium, 20% to 50% silicon and 3% to 10% aluminium.

Where heating is carried out in two stages the activating substance may be largely dispersed from the powdery mixture lduring the rst stage, thus necessitating the use of a fresh supply of powdery mixture foi the second stage. The powdery mixture may in that case be 0f the s-ame composition for both stages, or it may be enriched in chromium for the rst stage, chromium being relatively easily dilfused, and enriched in silicon for the second stage.

EXAMPLE 1 This example is of a method of manufacture which involves a single stage of heating.

A sheet metal specimen 0.05 inch thick made from an alloy consisting, Iby weight, of 10% tungsten and the remainder principally tant-alum, was subjected to a preliminary vapour blasting operation with a mild labrasive to obtain a ne matt o-r Satin finish and then chemically cleaned in a vapour degreasing plant, using trichloroethylene as the -active agent, until all condensation of Vapour on the specimen had ceased.

The specimen was then immersed in a powdery mixture contained in a refractory container made of sillimanite Iand having a loose fitting lid to reta-rd escape of the activating halide. The mixture consisted of 30% by weight of alumina, of approximately mesh per inch particle size, and 70% by weight of va reactive powder consisting by weight of 47.5% chromium, 47.5% silicon and 5% aluminium, the particle size in each case being up to 300 mesh per inch, .and to the whole there was added 1J/2% by weight of ammonium chloride.

The container was vibrated to ensure that the powdery mixture settled down around the specimen with exclusion of as much air Ias possible, and was then placed in a cold retort which was sealed except for a small gas escape hole and connected by piping to a cylinder of compressed argon gas, an adjustable pressure reducing valve being included in the system. Argon gas was allowed to iow through the retort at a sucient :rate to maintain a slight positive pressure in the retort, and after a period of 30 minutes, sufficient to ensure removal of substantially all the yair from the retort and from the interstices of the powdery mixture in the container, the retort was placed in `a furnace which had been raised to a temperature of 1350 C., the supply of argon being continued. After heating for four hours at this temperature the retort was removed from the furnace and allowed to cool to a temperature -at which it could be handled. The supply of argon was then shut olf, the container removed from the retort and the specimen taken out and cleaned lby brushing Off the loose powder and washing in hot water.

This treatment was found to have produced on the specimen a modied surface layer approximately 0.002 inch thick comprising chromium, silicon and aluminum alloyed with the tantalum alloy metals,

In the presence of an excess of the activating substance the thickness of the modied layer is dependent upon the temperature and time of heating, a lower temperature calling for a longer treatment time -and vice versa. Treatment temperatures Wholly below 1300 C. lare considered uneconomic in view of the prologation of the treatment time, While temperatures in excess of 1400 C., show insuicient advantage to compensate -for the more arduous operating conditions of the furnace and other equipment.

The degree of protection afforded to. the sheet metal specimen is dependent on the thickness of the modied layer and on the lrelative proportions of the infused chromium, silicon and aluminium in the modied layer. Since these materials have different infusion rates, their proportions in the modified layer `are not the same as in the powdery mixture, but nevertheless their proportions in the modified layer can be controlled 'by varying their proportions in the powdery mixture. The proportions at present preferred for the reactive content of the powdery mixture are, by weight, 47.5% chromium, 47.5 silicon and 5% aluminium.

The treatment is applicable to articles made from other alloys containing more than 50% by weight of tantalum than the one specifically mentioned in the example.

Articles according to the invention may be made still further resistant to oxidation by applying a surface glazing of a glass-like material of composition appropriate t the working temperature.

EXAMPLE 2 This di'ered basically from the rst example in that heating was carried out in two stages.

During the first stage the temperature was 1 180 C. and during the second stage it was 1350* C. Argon was used as the permeating gas for both stages. The period of heating during each stage Was 3 hours. A fresh supply of powdery mixture was used for the second stage. The composition of the reactive content of the powdery mixture during both stages was 47.5 chromium, 47.5% silicon, and 5% aluminium. The reactive content was mixed with alumina in the proportion 30% by weight of alumina to 70% by weight of the metal powder. The activating material was ammonurn chloride constituting 11/2% of the total weight of the powdery mixture.

This treatment gives a coating providing good protection for about 500 hours at 1150 C. or for about 200 hours -at 1300 C.

When the heating is carried out in two stages, as in the case of Example 2, the composition of the reactive part of the powdery mixture for the first stage may be 70% chromium, 25 silicon and 5% aluminium, and for the second stage the composition may fbe 25% chromiurn, 70% silicon 'and 5% aluminium. The temperatures for the two stages may be the same as in Example 2, but the period of the first stage is preferably increased to 3 to 4 hou-rs and that of the second stage reduced to 2 to 3 hours.

We claim:

1. A method of making an article wherein an article made from a body of refractory metal in which tantalum constitutes at least 50% by weight is heated in contact with a powdery mixture containing silicon, chromium and aluminium in powder form in admixture with a non-reactive refractory dispersant and an activating substance selected from the group consisting of ammonium bro.- mide, vvammonium chloride and ammonium iodide, the time and temperature of the heating being suicient to form on the article a modied surface layer at least 0.001 inch thick containing chromium, silicon and aluminium alloyed with the body metal, and the powdery mixture being permeated with an inert gas throughout the heating, the reactive content of the powdery mixture comprising, by Weight, 20% to 75% chromium, 20% to 75% silicon and 3% to 25% aluminium, the particle size in each case being up to 300 mesh per inch.

2. A method according to claim 1 in which heating is carried out in two stages, the temperature being 1100 C. to 1200" C. during the rst stage and l300 C. to 1400 C. during the second stage, the reactive content of the powdery mixture being relatively rich in chromium during the rst stage of heating yand relatively rich in silicon during the second stage.

3. A method according to claim 1 in Which the reactive content of the powdery mixture comprises :between 3% and 10% by weight of aluminium.

4. A method according to claim 1 in which the non-reactive refractory dispersant in the powdery mixture is alumina having a particle size of from 300 to. mesh per inch, the alumina being present in a quantity between 25 and 70% *by weight of t-he powdery mixture.

5. A method according to claim 4 in which the alumina is present in a quantity between 25 and 50% by weight of the powdery mixture.

6. A method according to claim 1 in which the activating substance is ammonium chloride, present to the extent by weight of from 0.5% to 2% of the total weight of the other ingredients of the powdery mixture.

7. An article produced by the method claimed in claim 1 comprising -a body of refractory metal in which tantalum constitutes at least 50% by weight and a surface layer containing only elemental chromium, silicon and aluminium alloyed with the body metal.

8. An article according to claim 7 in which the modified layer is at le'ast 0.001 inch thick.

References Cited UNITED STATES PATENTS RALPH S. KENDALL, Primary Examiner.

U.S. Cl. X.R 

